In the paper of Bloch and Kato in the Grothendieck Festschrift, and some other papers relating to the Bloch-Kato conjecture and the ETNC, the cohomology groups

$H^i_{\mathrm{et}}(\operatorname{Spec} O_{K, S}, M),$

seem to come up often, where $K$ is a number field, $S$ is a finite set of places of $K$, and $M$ is a finite or profinite $G_K = \operatorname{Gal}(\overline{K} / K)$-module unramified at primes outside $S$.

How should one think about these cohomology groups? How are they related to the much more familiar (to me at least) continuous Galois cohomology groups $H^i(G_K, M)$ (or the restricted ramification analogues $H^i(G_{K, S}, M)$)? Why are they the more natural things to work with in this context?

One thing is that Galois cohomology, by its very definition, requires the choice of a base-point (the `biggest' extension in which the primes outside $S$ are unramified), while etale cohomology is defined intrinsically in terms of the etale site over $\text{Spec}\mathcal{O}_{K,S}$. Usually, when you actually compute things, you end up returning to group cohomology, but it seems nicer to set up the theory without it.
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Keerthi Madapusi PeraApr 22 '11 at 13:44

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There is quite a bit on this in the second chapter of Milne's Arithmetic Duality Theorems --- see for example Proposition 2.9, the discussion on pages 195--197 (of the second edition), and Lemma 5.5.
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mephistoApr 22 '11 at 14:46

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There's an old paper of Mazur that discusses these kinds of groups, though it's been a long time since I looked at it so I don't recall what the specific goals of the paper are. I think it's called "Notes on the etale cohomology of number fields." It might be worth a look.
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RamseyApr 22 '11 at 16:29

@Keerthi: I think that Serre in his book on Galois cohomology talks about how one can do Galois cohomology "intrinsically"---probably all that it boils down to though is that he's doing etale cohomology really :-). Serre defines $H^i(k,M)$ without making a specific choice of alg closure of $k$.
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Kevin BuzzardApr 22 '11 at 21:32

1 Answer
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By usual (sometimes not so trivial) homological arguments, one can reduce to the case where $M$ is a finite discrete module over an artinian ring of residual characteristic $p$. In that case, I think you want $S$ to contain places above $p$ as well, even if your $M$ is unramified at $p$, so let me assume this.

The module $M$ induces an étale sheaf $M_{et}$ on $\operatorname{Spec}\mathcal O_{L,S}$ for all finite extension $L/K$. The spectral sequence UPDATE (converging to $H^{i+j}(\operatorname{Spec}\mathcal O_{K,S},M_{et})$)
$$E_{2}^{i,j}=\underset{\longrightarrow}{\operatorname{\lim}}\ H^{i}(\operatorname{Gal}(L/K),H^{j}(\operatorname{Spec}\mathcal O_{L,S},M_{et}))$$
then induces isomorphisms between $E_{2}^{i,0}$ and $H^{i}(\operatorname{Spec}\mathcal O_{L,S},M_{et})$ or in other words $H^{i}(G_{K,S},M)$ is isomorphic to $H^{i}(\operatorname{Spec}\mathcal O_{K,S},M_{et})$. So you can assume that you are working with Galois cohomology throughout $provided$ you use Galois cohomology with restricted ramification.

Because the Tamagawa Number Conjectures are formulated only in the setting above, Bloch and Kato could have used Galois cohomology instead of étale cohomology everywhere without changing anything. To touch upon your last question, I think there are two reasons why they chose étale cohomology.

First, at least at the time they wrote, Galois cohomology was not the most familiar object of the two. In fact, many classical well-known results were given correct complete proofs only very late (in the late 90s in some cases). On the other hand, SGA (and works of Bloch and Kato themselves) existed as references for étale cohomology.

Second, using étale cohmology, one can formulate the TNC over more general bases than $\operatorname{Spec}\mathcal O_{K,S}$ (for instance any scheme of finite type of $\mathbb Z[1/p]$). This kind of generalization had been the key idea of previous works of Kato and Bloch-Kato on higher class field theory so it is not surprising that they decided to at least allow the same kind of generality in their subsequent works.

"In fact, many classical well-known results were given correct complete proofs only very late (in the late 90s in some cases)." Which results are you thinking of?
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mephistoApr 22 '11 at 21:49

This is probably just me being slow, but could you elaborate a bit more on the spectral sequence argument? (For instance, what is this spectral sequence converging to?)
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David LoefflerApr 23 '11 at 7:11

@Mephisto Central to the approach of Kato is the fact that the étale cohomology of a perfect complex of étale sheaves is a perfect complex. In the early 90s, I wouldn't know what source to quote for the corresponding statement in Galois cohomology. But even something as "basic" as Poitou-Tate had no clear reference before the first edition of Milne's ADT, and that's only from 1986.
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OlivierApr 23 '11 at 7:32

Or think of the excision map in Galois cohomology with restricted ramification.
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OlivierApr 23 '11 at 8:30

@Loeffler. The spectral sequence and a proof of its degeneration can be found in Milne's ADT II Proposition 2.9. @Olivier. The results you mention are hardly "classical Well-known results". As far as I know, all such results had correct complete proofs in the literature by the mid-80s or earlier (not late 90s). For example, Poitou-Tate is proved in Haberland 1978. By contrast, the first complete proof in the literature of the very basic Artin-Verdier duality theorem is in ADT (1986), but that contained an error which wasn't fixed until the second edition (2004).
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mephistoApr 23 '11 at 11:17